From https://www.inaturalist.org/pages/help#quality:
“The system will vote that the observation is not wild/naturalized if there are at least 10 other observations of a genus or lower in the smallest county-, state-, or country-equivalent place that contains this observation and 80% or more of those observations have been marked as not wild/naturalized.”
In this case there was a group recording captive mussels at the Haley Institute of Animal Science and Aquaculture, so there are already a bunch of captive observations in this county – and not very many wild.